Don’t be scared of talking to someone you don’t know or want to approach - the chances are that they don’t like approaching people either.
Smile.
Decide what it is that you’d like to ask someone for - and make sure you get a chance to ask them!
Make sure you get people’s names and contact details - and that you have some manner of giving them yours.
If someone doesn’t have time or the inclination to help - ask them if they know someone who does.